Introduction to Fungi, Third Edition

396 HYMENOASCOMYCETES: ERYSIPHALES
About 15 min later, a further batch of matrix
material is released from the conidium, but
unlike the initial secretion this second wave
requires de novo protein biosynthesis. Within
2 h, a primary germ tube emerges. In all powdery
mildews except B. graminis, this develops an
appressorium under suitable conditions, but in
B. graminis the primary germ tube grows only to
a limited distance (up to 10 mm). An appressorium
is formed by a separate secondary germ tube
which is much longer than the primary germ
tube (up to 40 mm) and becomes septate. If the
primary germ tube fails to make contact with
a suitable surface, further short germ tubes may
be emitted. Upon contact with the host surface,
the primary germ tube secretes an adhesive pad
which provides more secure anchorage to the
surface. By suspending conidia on a spider’s
thread over different kinds of surface, Carver
and Ingerson (1987) observed that a long appressorial
germ tube is emitted only if the primary
germ tube has made contact with an inductive
surface such as a host leaf. The primary germ tube
therefore functions as a probe. Signals perceived
by the primary germ tube may be the hydrophobicity
of the surface, or minute quantities of
cutin or cellulose degradation products released
by appropriate hydrolytic enzymes which are
secreted by the primary germ tube (Green et al.,
2002). The signals are probably transduced by
cascades containing cAMP and cAMP-dependent
protein kinase A (cPKA), similar to those described
in more detail for Magnaporthe grisea (Fig. 12.48).
The primary germ tube may penetrate the surface
of the epidermis to a limited extent but does not
achieve successful infection of the host cell. It
may, however, take up water and dissolved substances
from the plant surface (Kunoh & Ishizaki,
1981; Carver & Bushnell, 1983).
Under optimal conditions, the appressoriumforming
germ tube emerges about 3 h after the
primary germ tube. It elongates and its tip
swells to form an appressorium which is lobed
(Fig. 13.4b) and non-melanized, in contrast to
the hemispherical melanized appressorium of
M. grisea described on p. 381. Appressoria
of other species of the Erysiphales may have
different shapes, and these are of taxonomic
significance (see Braun et al., 2002). The sequence
Fig13.4 Penetration events in Blumeria graminis. (a) Conidium
incubated on a cellulose membrane for12 h. After contact of
the primary germ tube (pgt) with the surface, the appressorial
germtube(agt)wasemittedandhasformedanincipient
appressorium. (b) A lobed mature appressorium on the
surface of a wheat leaf. (a) reprinted from Carver et al. (1999),
with permission from Elsevier; original print kindly provided
byT.L.W.Carver.(b)reprintedfromHoward(1997),withkind
permission of Springer Science and Business Media; original
print kindly provided by R. J. Howard.
of infection-related morphogenetic events up to
this stage is shown in Fig. 13.4.
Penetration is achieved by means of a thin
penetration peg which originates from the underside
of the appressorium. In all probability, both
the activity of secreted wall-degrading enzymes
(cutinases and cellulases) and appressorial turgor
pressure contribute to successful penetration
events, with the former predominating (Edwards
& Allen, 1970). Penetration can be achieved within
about 12 h of the conidium landing on the